1. Field of the Invention
The present invention relates to detecting the presence of developing faults in rolling element bearings by measuring the vibration of an apparatus in which bearings are operating and identifying signatures unique to bearing faults. These signatures are predicted by a fault signature model and recognized by a detector.
2. Brief Description of the Related Art
There is an abundance of techniques in the literature designed to detect the characteristic fault frequencies produced by faults in rolling element bearings. A bearing fault will produce one of the four characteristic fault frequencies depending on which bearing surface contains the fault. The characteristic fault frequencies can be calculated using equations (1)-(4), shown below, as described in T. A. Harris, Rolling Bearing Analysis 4th ed., New York: John Wiley and Sons Inc., 2001. pp. 307-311.
                              F          CF                =                              1            2                    ⁢                                    F              R                        ⁡                          (                              1                -                                                                            D                      B                                        ⁢                                          cos                      ⁡                                              (                        θ                        )                                                                                                  D                    P                                                              )                                                          (        1        )                                          F          ORF                =                                            N              B                        2                    ⁢                                    F              R                        ⁡                          (                              1                -                                                                            D                      B                                        ⁢                                          cos                      ⁡                                              (                        θ                        )                                                                                                  D                    P                                                              )                                                          (        2        )                                          F          IRF                =                                            N              B                        2                    ⁢                                    F              R                        ⁡                          (                              1                +                                                                            D                      B                                        ⁢                                          cos                      ⁡                                              (                        θ                        )                                                                                                  D                    P                                                              )                                                          (        3        )                                          F          BF                =                                            D              P                                      2              ⁢                              D                B                                              ⁢                                    F              R                        ⁡                          (                              1                -                                                                            D                      B                      2                                        ⁢                                                                  cos                        2                                            ⁡                                              (                        θ                        )                                                                                                  D                    P                    2                                                              )                                                          (        4        )            
These frequencies are illustrated in FIG. 1 where:
FR=rotor (shaft) frequency
FCF=cage fault frequency
FIRF=inner raceway fault frequency
FORF=outer raceway fault frequency
FBF=ball fault frequency
DB=ball diameter
DP=pitch diameter
NB=number of rolling elements
FRE=direction of force exerted by the rolling element on the outer raceway
θ=ball contact angle.
This invention claims that simply searching for the characteristic fault frequencies (either in the baseband vibration or through a high frequency resonance/envelope analysis technique) is often insufficient for the following two reasons. First, the frequency response of the apparatus being monitored will change continuously as a function of changes in load torque, coupled loads, mounting tightness, etc. A common consequence of this is to damp vibrations at the lower frequency ranges where the characteristic fault frequencies are predicted to appear. Therefore, even though these characteristic fault frequencies are produced inside a bearing with a developing fault, this energy is attenuated as it propagates through the apparatus toward the sensor and is thus undetectable.
Second, the vibration spectrum of any sufficiently complex (and actively excited) mechanical system is extremely rich with peaks. These peaks originate from a wide variety of sources including natural mechanical resonance, active sources from other mechanically and acoustically coupled systems, measurement noise, etc. Therefore, when searching for energy at a particular frequency or peaks at a given frequency spacing, it is common to find some amount of energy at these locations regardless of what the location is. However, it is impossible to know if these peaks are generated by the desired process (e.g., a bearing fault) or if they are from multiple unrelated processes and their location in the frequency spectrum is only coincidental. This invention describes a technique that successfully detects developing bearing faults while accounting for both of these phenomena.
As such, there is a need in the art to provide improved comprehensive detection of developing roller element bearing faults for operational machinery. The present invention addresses this and other needs.